Rachel Mills

December 2, 2008

Arc GIS

Term Project

Water-Use History in the San Antonio and Guadalupe River Basins

Introduction

One of the largest impacts on the global water cycle today is land use and land cover change (collectively referred to as LULC). It has a direct effect on the ecological environment and is considered one of the most important factors of global climate change (Vitousek 1993). Naturally, water has many pathways to enter the soil, but when vegetation is removed and replaced with concrete, all pathways are stopped and water will move horizontally until it reaches a permeable area. Increases in urban area from farming land have been shown to increase the volume of storm water run-off by up to 69% (Wegener 2001). Urbanization can also lead to “flashier” floods and higher levels of pollution. Stream quality often begins to degrade when impervious surface area within the contributing watershed exceeds 10% (Wolter et al 2006). Another impact of LULC is the removal of vegetation with no replacement (the land is left fallow), which leads to elevated rates of erosion. Coastal areas can be greatly affected by increased sediment loading in streams as it blocks sunlight, inhibiting photosynthetic activity. When areas are converted to agricultural land, excess fertilizers and herbicides can leach into the ground water and surface water, causing harmful algal blooms and clogging of the aquatic ecosystem.

Simply, when the native vegetation of an area is removed and replaced with something else, it will have a direct effect on the availability of water. Evapotranspiration is a process of removing water from the Earth’s surface through passive diffusion from plant’s stomata. Whenever the vegetation dynamics are changed, so is this process. It has even been found that LULC can regionally and locally affect climate, mainly by the changes in solar radiation absorption. A recent simulation suggests that converting an Amazon rainforest to pasture land would cause an increase in temperature, a decrease in precipitation, and it would also change the patterns of atmospheric circulation (Dickenson 1991).

Precipitation is a four step process involving first, the cooling of air to the approximate dew point. Then, condensation must occur, followed by growth of the rain droplets. Finally, a constant influx of vapor must be present to continue the process. When precipitation reaches Earth’s surface it can directly enter surface water bodies upon contact, land on impervious material and become overland flow, or infiltrate soil. In the soil, water can follow multiple paths including: being incorporated into vegetation through root uptake, being horizontally transported through capillary forces, or continuing downward towards a water table. The amount of precipitation received by an area can vary by location, time of year, and among years. Annual tree rings reflect the cyclic nature of a dry and wet season within each growing year. Apart from the normal precipitation variation within years, tree rings can also reflect variation of precipitation among years. Areas can experience draughts for multiple years followed by multiple years of heavier annual precipitation. The variance in precipitation among years will affect the amount of water available for direct use.

River discharge is the streamflow rate at a particular point in space. It is often influenced by precipitation rates. For example, the average discharge in the Guadalupe River for the summer of 2008 was 450cfs. One week after Hurricane Dolly passed through Southern Texas, the river discharge increased to 1100cfs within 12hours. This was a direct response to the extreme amount of precipitation experienced during the storm event. Like precipitation, low river discharge could be an indication of low water availability in the area.

As land use and land cover change over a landscape, so does the demand for water from vegetation cover and humans. Water-use is influenced by myriad parameters such as: population size, annual precipitation, composition of vegetation, level of urbanization, humidity, temperature, and topography. Historical water-use data could corroborate LULC findings as highlight the effect of LULC on the availability and demand for water.

The objective of this study was to investigate land use and land cover change in the San Antonio and Guadalupe River basins. Four parameters were chosen as representatives of inputs and outputs of the two river basins: precipitation, river discharge, land-use/land-cover change (LULC), and anthropogenic water-use. The San Antonio and Guadalupe River basins were chosen for this project, as part of an ongoing study, investigating storm events and their impact on the nutrient cycling and movement of water through the San Antonio and Guadalupe Rivers.

Both rivers begin in East Central Texas and drain into the San Antonio Bay, which empties into the Gulf of Mexico. The San Antonio River begins in Bandera County and it joins the Guadalupe River in Refugio County. The Guadalupe River begins north of the San Antonio River in Kerr County.  Both rivers are influenced by anthropogenic disturbance. The San Antonio River, unlike the Guadalupe River, runs directly through the third largest city in Texas, San Antonio. Within the city the river is a major tourist attraction as a “River Walk.” Portions of the Guadalupe River are also major tourist attractions for water sports such as river floating and boating.  Both rivers attract many fishermen as well.

San Antonio River Basin

Length = 240 miles

Drainage Area = 234 miles²

Average Discharge = 400 cfs

Guadalupe River Basin

Length = 230 miles

Drainage Area = 2023 miles²

Average Discharge = 1200 cfs

Precipitation:

Overall, there appears to be no specific pattern in the precipitation data through the years. Usually, a year of high precipitation is followed by several years of dryer conditions, until this repeats again. Not surprising, but the summer moths tend have less precipitation throughout the years and the winter/spring months tend to have more precipitation.

These two graphs for January and February are examples of the 12 months, to illustrate the fact that there is no trend (R²<0.50) indicating that the San Antonio and Guadalupe River basins are getting wetter or drier through the years.

Wettest Months

Jan

feb

march

april

may

june

july

august

sept

oct

nov

dec

1

1934

1958

1997

1976

1981

1973

2007

1914

1967

1960

2004

1923

2

2007

1923

1957

1957

1972

1977

1942

2001

1971

1981

1934

1991

Driest Months

1

1918

1916

1916

1987

1998

1967

1957

1930

1940

1952

1950

1917

2

1971

1925

1971

1984

1912

1980

1938

1927

2008

2008

1970

2008

The data was organized to find the top two wettest and driest years of each month to look for a pattern. Again, there doesn’t seem to be one. Some years have shown up, but it is interesting to note that the great draught in 1956-7 doesn’t appear frequently in the chart or precipitation graphs. Actually, 1957 is the second wettest month for March and April.

Discharge:

Overall, discharge appears to increase with year, but when analyzed the R² value is very small (0.17). The draught of 1956-7 is reflected in the discharge data at the point where discharge is almost reduced completely to zero.

Annual river discharge data was obtained from the USGS Water Watch website. I then plotted the data using Microsoft excel. There appears to be a moderate upwards trend in the discharge data, but it has a very small R² value of 0.07. This might be the result since there appears to be larger differences in discharge among years as the dates become more recent. This would cause greater variation and a smaller correlation. Overall, San Antonio and Guadalupe River discharge follow the same pattern through the years. It appears that the presence of a major urban development on the San Antonio River, the city of San Antonio, doesn’t appear to increase the relative discharge of the river through the years. It would be interesting to investigate the actual response time of each river after a storm event to see if the urban development along the San Antonio River does increase the “flashiness” of the river’s response.

When the two rivers’ annual discharges where compared, there seemed to be a fairly strong correlation between the two (R²=.82). Although the discharge values are highly correlated, there is still some variation among the two rivers. They receive the similar precipitation rates due to their close proximity to one another, so it does not appear to explain the observed variation.

I created this chart of mean annual flow using the Arc GIS Hydro program. I used NHD streamflow data and NHD catchment information to create the image. The San Antonio and Guadalupe Rivers are easily distinguished as the two thickest lines than run along the northern and southern edges of the basin’s border. Although it isn’t easy to distinguish in the chart, the Guadalupe River does have a higher mean annual flow than the San Antonio River.

Land Use Land Cover Change (LULC):

The southern end of the basin is dominated by agricultural use, while the northern half is dominated by forest and grassland/shrub. The large red circle in the middle of the San Antonio River is the city of San Antonio. This is the largest region of urban development in the entire San Antonio/Guadalupe basin. The far southwest corner of Travis County is located in the Guadalupe basin, which includes the city of Austin. This could have a small effect on the response to precipitation events through increased overland flow.

From 1992-2001, there appears to be minimal change to the land cover in the San Antonio and Guadalupe River basins. The northern half of the basin is speckled with pockets of small changes (green). This is most of the land near San Antonio, San Marcus, and other cities along the way which might be a driving force in land cover change. In the southern half of the map there are sprinkles of even larger land cover change (blue). This area is dominated by agricultural use and this change could be remediation of agricultural land or the creation of it.

Category

1992

2001

Open water

183865

204403

Urban

2322486

2568680

Barren

37931

52851

Forest

7819649

6933025

Grassland/Shrub

10828073

11292678

Agriculture

7296467

7300536

Wetlands

826000

962298

The above table reiterates the fact that there was minimal land use land cover change from 1992-2001 in the San Antonio Guadalupe River basin area. Overall, the land cover percent change was 6.45%.

Category

1992

2001

Open Water

115340

117095

Urban

919983

992306

Barren

12779

18718

Forest

4860565

4321470

Grassland/Shrub

6634737

7001212

Agriculture

4137028

4165821

Wetlands

584855

640933

The overall percent land cover change for the Guadalupe River basin is 5.2%. The largest change in land cover type is an 11% reduction of forest land and a 5% increase in the amount of grassland/shrub from 1992-2001. The LULC shown in this picture corroborates the results of the LULC calculations for the entire Guadalupe/San Antonio River basin area. The largest amount of land cover change occurs in the north central part of the basin. There is hardly any land use change in the southern portion of the basin. This area is dominated by agricultural use and is most likely still in production hence the minimal land use change. This is also reflected in the minimal increase in agricultural land.

Category

1992

2001

Open Water

68525

79576

Urban

1402471

1576338

Barren

25156

34133

Forest

2959002

2611501

Grassland/Shrub

4193136

387852

Agriculture

3159903

3135168

Wetlands

241118

321340

The overall percent land cover change for the San Antonio River basin is 8.2%, which is the largest percent change of the two basins. This is also illustrated in the above figure with the extensive distribution of red and blue dots in the watershed. The largest change in land cover was a 12% reduction in forest land cover and a 9% reduction in grassland/shrub cover from 1992-2001. In the northern region of the basin there is a circle of minimal land cover change surrounded by an outward moving ring of land cover change. The circle is the city of San Antonio and most likely the land cover change seen in the analysis is the result of urban sprawl and increased sub-urban development. The extent of the land cover change in the San Antonio area is actually less than in the southern portion of the basin, which is covered in high land cover change values. This portion of the basin is dominated by agricultural land cover, but result of this analysis show a decline in the agricultural land cover. The land use figure shows a large portion of this basin covered in barren land. The reduction in agricultural land in the southern basin might be the result of land taken out of production to lay fallow.

Water Use:

Data was collected from a representative at the TCEQ, who helped compile the data. Unfortunately, water-use applications are voluntary and are not considered very accurate. I thought it would be good to initially investigate the possibility that the water-use values were highly correlated with the number of applications submitted in that year.

There doesn’t seem to be a strong correlation that the number of applications correlates with the water-use amount reported (R²=0.38).

In the early 1940’s there is a huge increase in the water-use reported to the TCEQ. Earlier, I showed that the number of applications did not correlate with water-use, therefore it must be assumed that the above graph is a reflection of increased water demand from both rivers. I am unsure, why the water-use has dropped around the late 1980’s.

Initially, San Antonio River’s water-use was very high and then it rapidly declined around 1921. Then it increased again and has roughly stayed at the same amount since then.

The Guadalupe River is almost opposite of the San Antonio in that it had a very small water-use initially and then it rapidly increased in the mid-1940’s (much like the total water-use graph above). In the late 1980’s there is a sharp decline in water-use that should be investigated further.

Water-use data from 1990 to present was reported in a different format than the other historical data and had to be analyzed separately. Many years had less than 10 applications per year and were omitted from analysis for this reason. Below are the only three years in this range with sufficient data.

From 1990-present the water-use amount continues to decline from the previous graph of total historical water-use. It is unclear what has caused the reduction in use since neither precipitation nor discharge data indicate limited water availability. Also, the land cover change data fails to suggest a large reduction in urban and agricultural land use, two primary water users in the area, which would also explain the decline in water-use. The overall water-use pattern tends to follow Guadalupe River, I have left the data as total water-use, instead of by river. Looking back at the precipitation data, May 1990 was drier that previous and following years, which explains the large amount of water used during that month. Water-use is actually negative for June 1993 and October 1998. Again, looking at the precipitation data, you can see that both of these months for those years were fairly wet, allowing companies to revert water back into the rivers.

Conclusions:

The San Antonio and Guadalupe River basins collectively experienced minimal land cover change over the nine years of data. The San Antonio River basin has the largest percent change of the two basins, although the change is still less than 10%. Both basins experienced the largest change as a reduction in forest land cover. The direction of change among the two basins was different for many of the land use categories, such as agriculture (increasing in the Guadalupe basin and decreasing in the San Antonio basin) and grassland/shrub (increases in the Guadalupe basin and decreases in the San Antonio basin).

Since the middle 1940’s, water-use greatly increased in the San Antonio and Guadalupe River steadily decreased. As mentioned above, the cause for the decline in use is unknown and requires further investigation. It should also be remembered that water use data was voluntary and it is possible that the numbers reported were inaccurate and much of the water used during those years has gone unreported. The demand for water is much higher in the Guadalupe River, which is also why the total water-use graph resembles the Guadalupe River graph so closely. This isn’t surprising since the Guadalupe has a significantly larger discharge. None of the water-use graphs resemble the (random) patterns of either the precipitation or discharge data, which suggests that precipitation and river water level are not the limiting factors that decided the amount of water to be used in a given year. This is somewhat surprising since wetter years/months would require less water inputs for irrigation, one of the largest water uses. What data is available for 1990-present does show that water-use does seem to follow precipitation data and this could be a recent trend to sustainably use water.

It is difficult to make any confident conclusions about the water-use of the San Antonio and Guadalupe River basins for a few reasons. First of all, there is no consistent pattern in precipitation, land use land cover change, or discharge to suggest that the availability or demand for water is the driving force of the increased water-use seen in this project. Secondly, it is still unclear what the water was actually being diverted for in the data given. Reading through the names of the companies in the water-use application data provides some insight into the fate of the water. It appears that a lot of the water reported was used for agricultural irrigation, but this cannot be confirmed at the moment. Thirdly, since the water-use applications are option it is impossible to know how much water-use each year is undocumented and of the water-use that is documented, how accurate are the values.

Future work should begin with an investigation into the companies listed in the water-use data tables to accurately assess where the water is going. This could also be done by assessing water-use by county and interpret water-use from the land cover map created for this report. Also, since precipitation seems to have minimal effect on the apparent water-use, it would be worthwhile to investigate other parameters that could influence water-use, such as topography, vegetation cover, and soil properties/chemistry.

Data Used:

- Precipitation Data from the National Climate Data Center,  http://cdo.ncdc.noaa.gov

- River Discharge Data from the USGS Water Watch,  http://water.usgs.gov/waterwatch

-NHD Streamflow data

- LULC Data from the Multi-Resolution Land Characteristics Consortium (USGS), http://www.mrlc.gov/multizone_map2.php

- Water Use Data from Texas Commission for Environmental Quality (TCEQ) (compiled by Peter Bohls)

Literature Used and Cited:

Dingman, S. Lawrence. Physical Hydrology (2nd ed.). Prentice Hall, New Jersey. 2002

Dickenson, R.E. 1991. Global change and terrestrial hydrology- a review. Tellus 43: 176-181.

Vitousek, P.M. Beyond global warming: ecology and global change. Ecology 75: 1861-1876.

Wegener, M.W. 2001 Wegener, M.W. 2001. Long-term land use/cover change

patterns in the Yahara Lakes region and their impacts on runoff volume to Lake Mendota. M.Sc. thesis, University of Wisconsin-Madison, Madison, Wisconsin.

Wolter, P.T, C.A. Johnston, and G.J. Niemi. 2006. Land use and land cover change in the US Great Lakes region. Journal of Great Lakes Research 32: 607-628.